The present invention relates generally to the reliability and restoration of optical transmission systems.
1. Field of the Invention
The present invention relates to communications networks. More specifically, the present invention relates to an apparatus and method for controlling restorative switching of an optical network.
2. Related Art
A service disruption in a communications network may be caused by a number of problems including inoperable communications links, cable cuts, or failure of equipment within a system. In the event of a failure the traffic must be restored temporarily until the failure is repaired. Two restoration approaches are network restoration switching and line protect switching. The restoration approach which is used varies depending on the failure.
Network Restoration Switching: When a fiber cut or other major failure disables a communications system, network restorative switching (NRS) may be employed to reroute the traffic through the network via a restoration path. The restoration path carries traffic between the two end nodes until repairs of a path normally supporting the system are complete.
A mesh topology is a preferred topology for a network using NRS. A properly equipped mesh network contains switching nodes that are connected to two or more adjacent nodes. In a mesh topology, a given signal may have many possible routes by which to traverse the network. Mesh networks allow for sophisticated actions to be taken in response to a failure. If there is a sudden failure of several links, or an entire span, neighboring switching nodes can perform distributed switching to divert the traffic around the failure. For this purpose, most of the spans in a mesh network are equipped with extra spare links that can be called upon for emergency traffic handling in response to a failure.
The manner in which a given network should switch to recover from any possible failure is presently the subject of considerable development in the communications field. One type of network restoration scheme locates a service disruption, identifies alternate routes, and then establishes such routes, in order that a service disruption will minimally affect a communication system user. Two types of control schemes for mesh restoration are a centralized scheme and a distributed pre-planned scheme.
In a centralized restoration scheme, a telecommunication network includes a central site capable of establishing alternate routes when a failure occurs. For an example of a centralized restoration scheme see U.S. Pat. No. 5,182,744 issued to J. Askew et al., incorporated in its entirely herein by reference.
In a distributed pre-planned restoration scheme, network connections are restored by intelligent switching nodes distributed throughout the network. For an example of a distributed pre-planned restoration scheme see U.S. Pat. No. 5,173,689 issued to T. Kusano, incorporated in its entirety herein by reference.
When a disruption occurs, the time to restore service using NRS depends upon a number of factors, such as (a) the time required to identify the locations of the service disruption; (b) the time required to determine alternative routes that avoid the service disruption; and (c) the time required to actually establish such routes. In selecting a new communications route, it is often desirable to select the most efficient alternate route, i.e., the one generally having the minimal distance and/or hop count.
Line Protect Switching: Another method of network restoration is line protect switching (LPS). In LPS, local light termination elements (LTE) reroute traffic from a xe2x80x9cworkingxe2x80x9d channel to a xe2x80x9csparexe2x80x9d channel. LPS ensures resiliency to equipment failure by employing a spare channel, also referred to as the protect channel, that normally does not carry traffic but may be used as a back-up should a xe2x80x9cworkingxe2x80x9d channel fail. The spare channel can be rerouted along the same physical path as the working channel. Preferably, the spare channel is routed along a completely different physical path to minimize the chance that the spare channel experiences the same fate that disabled the working channel.
In order to reduce costs, LPS can employ the use of one spare channel for five working channels. Because fewer spare channels are available than working channels, LPS cannot restore an extensive failure, such as a cable cut. LPS is primarily aimed at restoring single channel failures and is implemented within the LTE, which is the local equipment that terminates the fiber optic cable. Since LPS is localized and simple, it is also very fast requiring only tens of milliseconds for restoring a failed communications system. LPS can quickly restore simple localized failures. In many applications, the LPS can switch traffic without causing any significant interruption to traffic. LPS is described in further detail in commonly-owned, co-pending U.S. patent application Ser. No. 08/672,808 entitled, xe2x80x9cSystem and Method for Photonic Facility and Line Protection Switching,xe2x80x9d filed by John Fee on Jun. 28, 1996, incorporated in its entirety by reference herein.
Because telecommunications networks can include high capacity terrestrial and under water optical cables, the networks are susceptible to failures that disable a very large number of channels causing potential high volume of traffic loss and significant economic impact. There is a need for a fast, efficient, and reliable apparatus and method for restoring a network. Additionally, there is a need for an apparatus and method for restoring a network that takes advantages of the benefits of both LPS and NRS.
The present invention provides an apparatus and method for restoring systems of an optical network. Each node in the optical network includes an optical cross connect switch (OCCS) and an OCCS-controller. OCCS-controllers control the switching of respective OCCSs to perform NRS in response to a failure notification.
Within a first end node, an OCCS and an OCCS-controller make up a first optical switching unit (OSU). Within a second end node, an OCCS and an OCCS-controller make up a second OSU. One of the two OSUs is designated the primary OSU while the other is designated the secondary OSU. In the event NRS is needed, each OSU independently and in parallel determines a restoration plan for providing a restoration path between the first and second end nodes. The primary OSU sends a primary restoration plan to OSUs in other nodes of the optical mesh network for effecting the restoration path. If the secondary OSU does not receive information identifying the primary restoration plan from the primary OSU, then the secondary OSU sends a secondary restoration plan to the other OSUs in the other nodes of the optical mesh network for effecting the secondary restoration path. In the preferred embodiment, the primary restoration plan is the same as the secondary restoration plan. By having both the primary OSU and secondary OSU determine restoration plans at the same time, the speed in which, and the likelihood that, a restoration path will be established is increased.
The first and second end nodes each also includes an LTE which communicates with their respective OCCS-controllers. The LTEs can perform LPS to switch to a spare channel when a working channel fails. The OCCS-controllers can switch the OCCSs to perform NRS. In the preferred embodiment, if the LTEs are unable to restore a failure, then the LTEs transfer responsibility for restoration to their respective local OCCS-controllers.
A feature of the present invention is that a primary restoration plan and a secondary restoration plan for restoring a network are determined independently and in parallel.
Another feature of the present invention is that network restoration switching (NRS) coexists with line protect switching (LPS) without causing oscillations between the LPS and NRS.
An additional feature of the present invention is that each OCCS-controller at an end node performs restoration functions independently of other end nodes so that a failure of several systems does not increase the time to restore the first failed system.
A further feature of the present invention is that a restoration plan chosen can be dependent upon the location of a failure within a path supporting a system.
Still another feature of the present invention is that a default restoration plan can be chosen if the location of a failure is unable to be isolated within a predetermined amount of time.
Further feature and advantages of the invention, as well as the structure and operation of the invention, are described in detail below with reference to accompanying drawings.